Don't be ridiculous. A standard and unmodified speedometer in a standard and unmodified car is fit for purpose.It,s your car, its your speedometer, its your responsibilty to ensure its fit for purpose.
You're getting confused with the "ignorance of the law is not a defence in law" mantra, which generally holds true.Ignorance of its faulty condition, is no excuse in law
If a speedometer becomes faulty, bearing in mind that it isn't examined as part of an MOT test, then it's possible to remain blissfully unaware until a speed camera (or police officer) alerts you of the situation. However, knowing how they work, and knowing that they over-read, I know that this must be a rare occurrence, so in a genuine case it would be reasonable to expect leniency, but not acquittal.